1. Field of the Invention
The present invention relates to an optical signal processing apparatus such as a magneto-optic disk apparatus for reading data recorded in, e.g., a magneto-optic disk.
2. Description of the Related Art
An optical disk apparatus or a magneto-optic apparatus is known well as a recording medium signal processing apparatus using a recording medium (disk). In such an apparatus, physically recessed areas called pits, areas in which magnetic characteristics are changed, or areas in which the state of a metal is changed are formed on data tracks called concentric or spiral grooves on a disk, and data is written (recorded) or read out (reproduced) by using these areas.
In a conventional apparatus in which data is recorded/reproduced along recording tracks, an optical head is controlled to form a light spot on a track. When data in a target track distant from a current position of the optical head is to be written or read, the optical head is moved to access the target track. This operation for accessing a track is called a seek operation.
In the seek operation, as disclosed in Published Unexamined Japanese Patent Application No. 2-14430, when a head carriage assembly having an optical head is moved in the radial direction of the disk while the speed of the head is controlled, a light spot from the optical head traverses recording tracks to quickly move to a target track.
Although the speed of the optical head must be detected to control the speed, in this case, the speed is detected as follows. A beam reflected by the optical disk is received by a four-divided photodiode for generating a tracking control signal. Electrical signals generated from the photodiode are added in adders and a difference between two signals from the adders is detected. A difference signal representing the difference is binarized by using a predetermined threshold level. A period of the binary signal is counted using a predetermined clock, and the reciprocal of the number of the count result is calculated, thereby detecting the speed of the optical head.
In order to cause the light spot to traverse recording tracks to quickly and reliably reach a target track, the moving direction (the direction from the inside to the outside of the disk or vice versa) of the optical head must also be detected during the control operation of the speed. A reason for this will be described below.
For example, when the light spot is moved from the inside to the outside of a rotating disk, and when the grooves on the disk is shifted in the same direction of the light spot at a speed higher than that of the light spot due to the eccentricity of the disk or the like, the traversing speed of the light spot relative to the grooves is temporarily reversed. That is, the moving direction of the light spot is reversed due to the eccentricity of the disk.
Therefore, in an apparatus using a disk having inevitable eccentricity to some extent, the moving direction of the optical spot must be quickly and reliably determined. This determination is a necessary condition for the head speed control operation.
The direction of the light spot is detected from the two signals generated from the adders. The direction is detected by binarizing the difference signal (to be referred to as a tracking difference signal) representing a difference between the two signals and a sum signal (to be referred to as a tracking sum signal) representing a sum of the two signals, and using a phase difference between the binarized difference signal and sum signal.
The signal level of the binary difference signal is held at a timing of the leading edge of the binary sum signal obtained while the optical head is moved, and the moving direction of the optical spot is determined by the level of the held signal. That is, when the held signal is at a high level, the light spot is moved from the outside to the inside of the disk; and when the signal is at a low level, the light spot is moved from the inside to the outside.
As described above, the speed is detected by using the tracking difference signal, and the direction is detected in accordance with the phase difference between the sum signal and the difference signal. Therefore, the speed can be accurately controlled, and a high-speed seek operation on the basis of a remaining track count to a target track can be realized.
When the optical head is moved over tracks in which data are written, a data signal is superposed on a tracking signal. For this reason, an accurate binary signal may not be obtained. Conventionally, the data signal in the tracking sum signal used in a seek operation is neglected. In this case, an accurate relative moving direction between the tracks on the disk and the light spot traversing the tracks cannot be detected, and the speed of the light spot cannot be easily controlled.